Propulsion system

ABSTRACT

An arrangement is disclosed for supplying propulsion power to a contra rotating propellers (CRP) propulsion system in a marine vessel, which arrangement can include a first propeller driven by a rotating power unit, and a second propeller driven by an AC motor. The second propeller can be rotated in an opposite direction relative to the first propeller, wherein an AC generator is driven by the rotating power unit and the AC generator is electrically connected to the AC motor. The rotation speed of the second propeller is between for example 95% and 150% of the rotation speed of the first propeller. The AC motor and the AC generator have the same electrical frequency. The pole number of the AC generator is from for example 2 to 40 and the pole number of the AC motor is from for example 2 to 40.

RELATED APPLICATION

This application claims priority as a continuation application under 35 U.S.C. §120 to PCT/EP2012/050022, which was filed as an International Application on Jan. 2, 2012 designating the U.S., and which claims priority to European Application 10197480.6 filed in Europe on Dec. 31, 2010. The entire contents of these applications are hereby incorporated by reference in their entireties.

FIELD

The present disclosure relates to an arrangement for supplying propulsion power to a contra rotating propellers (CRP) propulsion system in a marine vessel, which arrangement can include a first propeller driven by a rotating power unit and a second propeller driven by an AC motor.

BACKGROUND INFORMATION

The propulsion power of large ships or marine vessels can be generated by a rotating power unit, whose energy source can be oil, gas, or nuclear power. The rotating power unit may be a diesel engine, gas turbine or nuclear power reactor rotating turbine. The mechanical output shaft is either directly or via a gear coupled to the shaft of the propeller, or the rotating power unit drives a generator, which supplies electric power to the propeller motors of the ship. Furthermore, several different combinations of these two main manners to drive the propeller have been utilized.

The power and energy efficiency of the marine vessel warrant that the propulsion power be generated as economically as possible in different operation modes. The overall energy consumption should therefore be optimized. That is, the electric energy should be generated using a most economical power production system that is available on board and the electric energy should be utilized as efficiently as possible when supplying the electric energy to the devices and motors using the electric energy.

As is known in the art, propulsion devices consume the majority of the energy expended in marine vessels. Further, the propulsive force or thrust is generated using simultaneously different kind of engines or motors. Therefore it is desirable that these devices cooperate so that when producing the desired propulsive power and steering their combined energy consumption is as low as possible. At the same time the total energy consumption and the generation of electricity should be as efficient as possible. It is desirable that the total energy of the vessel be generated effectively and the electric power be supplied for use by all the electric power consuming devices effectively in different operating situations.

Publication WO 02/072418 discloses a CRP-propeller arrangement where one propeller is driven by a main engine and another propeller is driven by an electric motor. The shafts of the first and second propellers are coaxial and the shaft of the first propeller is arranged in the hollow shaft of the second propeller. The main engine drives the first propeller coupled to the shaft of the main engine and the generator arranged relative to the shaft of the main engine supplies electric power to a motor that drives the second propeller. In addition, there are other engines that rotate generators that supply power to the motor.

Publication DE 3207398 discloses a CRP arrangement wherein the main engine rotates on a propeller arranged on the hull of the ship and another propeller is arranged relative to the rudder unit. The other propeller is rotated by a motor that is supplied from a generator driven by the main engine. The motor is connected directly to the generator or via a frequency converter. Alternatively the motor is supplied via a main switch board from another generator driven by another main engine.

SUMMARY

An arrangement is disclosed for supplying propulsion power to a contra rotating propellers (CRP) propulsion system, which arrangement comprises: a rotating power unit for driving a first propeller; an AC motor for driving a second propeller, whereby the second propeller is configured for rotation in an opposite direction relative to rotation of the first propeller; and an AC generator configured to be driven by the rotating power unit, the AC generator being electrically connected to the AC motor, wherein a rotation speed of the second propeller will be between 95% and 150% of a rotation speed of the first propeller, wherein the AC motor and the AC generator have the same electrical frequency, and wherein a pole number of the AC generator is from 2 to 40, and a pole number of the AC motor is from 2 to 40 so that a ratio of the pole numbers of the AC motor and the AC generator is from 0.05 to 20.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will be described in more detail in the following by referring to the appended figures, in which:

FIG. 1 shows an exemplary embodiment as disclosed herein;

FIG. 2 shows another exemplary embodiment as disclosed herein;

FIG. 3 shows an exemplary operation mode of the exemplary FIG. 2 embodiment;

FIG. 4 shows an exemplary second operation mode of the FIG. 2 embodiment;

FIG. 5 shows an exemplary third operation mode of the FIG. 2 embodiment;

FIG. 6 shows an exemplary fourth operation mode of the FIG. 2 embodiment;

FIG. 7 shows an exemplary fifth operation mode of the FIG. 2 embodiment;

FIG. 8 shows another exemplary embodiment as disclosed herein; and

FIG. 9 shows another exemplary embodiment as disclosed herein.

DETAILED DESCRIPTION

A new and cost-effective arrangement is disclosed to supply electric power in a ship or marine vessel.

An exemplary arrangement for supplying propulsion power to a contra rotating propellers (CRP) propulsion system in a marine vessel can include a first propeller driven by a rotating power unit and a second propeller driven by an AC motor, whereby the second propeller is rotated in an opposite direction relative to the first propeller. An AC generator is driven by the rotating power unit and the AC generator is electrically connected to the AC motor. The rotation speed of the second propeller is between, for example, 95% and 150% of the rotation speed of the first propeller. The AC motor and the AC generator can have the same electrical frequency. The pole number of the AC generator is from, for example, 2 to 40, and the pole number of the AC motor is from, for example, 2 to 40 so that a ratio of the pole numbers of the AC motor and the AC generator is from, for example, 0.05 to 20. The value of the pole number is an even number.

According to an exemplary embodiment, the diameter of the second propeller is smaller than the diameter of first propeller.

According to an exemplary embodiment, a distance between the first propeller and the second propeller is less than twice the diameter of the first propeller.

According to an exemplary embodiment, a power proportion of the first propeller and the second propeller ranges between, for example, 50:50 and 80:20.

According to an exemplary embodiment, the AC motor is supplied by a second electrical power source, which is electrically connected in parallel to the AC generator.

According to an exemplary embodiment, the AC generator supplies electric power to a second electrical consumer, which is electrically connected to the AC generator.

According to an exemplary embodiment, the second power source includes a frequency converter, and the second power source is electrically connected via a switch to the AC motor.

According to an exemplary embodiment, the second power source is switchable to the AC motor and/or the AC generator when changing the power distribution between the first propeller and the second propeller.

According to an exemplary embodiment, an electrical power connection between the AC motor and the AC generator includes a line breaker, and the AC generator and the second power source are alternatively switchable to the AC motor when maneuvering the vessel.

When using exemplary arrangements as disclosed herein, there is more freedom in a ship's general arrangement and hull design as compared for example to the mechanical propulsion. The electric power generators can be positioned in a more advantageous way and the hydrodynamic efficiency of the vessel can be thereby improved.

The energy can be generated by a most efficient way in different operation situations and when fulfilling varying power demand of the vessel. Thereby the electrical losses of the marine vessel can also be minimized as compared to known electrical propulsion systems.

The exemplary arrangements as disclosed herein can enable a configuring of power generating plants and the engines using a primary energy source in a sensible way in accordance with the specifications of the vessel. This gives freedom to choose the most suitable of the plurality of multiple power plant configurations and thus enables energy-efficient ship operation.

The energy generating machines such as diesel engines, and the propulsive power generating motors such as electric motors or main propeller driving diesels are distributed on the vessel and they can be connected and disconnected. Thereby the redundancy of the propulsive units or their subunits can be increased.

FIG. 1 shows a schematic diagram of an exemplary embodiment disclosed herein, wherein a main rotating power unit 2 that is for example a two-stroke diesel engine drives a propulsion AC generator 4, whose rotor is fastened to the output shaft of the rotating power unit 2. A main propeller 6 of the vessel is fastened onto another output shaft 10 of the rotating power unit 2. Though the AC generator 4 and the propeller 6 are on the opposite sides of the rotating power unit 2 in the embodiment shown in FIG. 1, the generator may also be on the same side of the rotating power unit 2 as the propeller 6.

Opposite to the main propeller 6 is arranged a second propeller 12 which is driven by an AC motor 8. The main or the forward propeller 6 is fitted with a bearing to the hull of the vessel. The second or the aft propeller 12 is supported with the AC motor 8 to the hull of the vessel or to the rudder arrangement of the vessel. The forward propeller 6 and the aft propeller 12 are arranged to operate in a contra rotating propeller (CRP) mode that is known to the those skilled in the art.

The AC generator 4 is connected to the AC motor 8 by an electrical power connection 14. The AC motor 8 and the AC generator 4 have the same frequency. Their rotational speed as well as the rotational speed of the aft and the forward propellers is affected by the frequency.

The power P_(ME) of the rotating power unit 2 can be some dozens of MW, such as 60 to 100 MW or more depending on the propulsion power demand of the marine vessel. The power P_(G) of the AC generator 4 and the power P_(M) of AC motor 8 can be in the range of for example 20 to 50% of P_(ME). Thus, the power proportion of the forward and aft propellers varies from for example 50:50 to 20:80.

The pole number of the AC generator 4 and the pole number of the AC motor 8 can have values that are chosen to give an optimal propulsion effect in the CRP-arrangement. The rotation speed of the second propeller is between for example 95% and 150% of the rotation speed of the first propeller. The pole number of the AC generator 4 may have a value between for example 2 to 40 and the pole number of the AC motor 8 may have a value between for example 2 to 40, whereby the value of the pole number is an even number. The ratio of the pole numbers of the AC motor and the AC generator is for example from 0.05 to 20.

The values can be defined case by case when dimensioning the whole CRP drive and several factors can be effected by it. There may be gears between the rotating power unit and AC generator, gears between the forward propeller and rotating power unit, and gears between the aft propeller and the AC motor. The diameter proportions of the propellers and distance between the propellers also affect what is the desired proportion of the rotational speed of the aft and forward propellers. By selecting the pole number of the AC generator and the pole number of the AC motor, the desired proportion of the rotational speed of the aft and forward propellers can be achieved as the mutual speed of the AC motors and the AC generator is defined by their pole numbers.

As an example: the rotational speed of the rotating power unit is 100 rpm and the AC generator and the forward propeller is directly connected to the shaft of the rotating power unit. It is desired that rotational speed of the aft propeller that is directly connected to the shaft of the AC motor should be 120 rpm. Then by selecting the pole number equal to 12 for the AC generator and the pole number equal to 10 for the AC motor the desired proportion for the rotational speeds of the propellers will be achieved as the AC generator and the AC motor are directly connected by the electrical power connection and they have the same frequency.

As another example: the rotational speed of the rotating power unit is 100 rpm of the rotating power unit and the forward propeller is directly connected to the shaft of the rotating power unit. The AC generator is connected via a gearbox to the shaft of the rotating power unit. A transmission ratio of the gear is equal to 3 and the rotational speed of the AC generator is equal to 300 rpm. It is desired that rotational speed of the aft propeller that is directly connected to the shaft of the AC motor should be 120 rpm. Then by selecting the pole number equal to 4 for the AC generator and the pole number equal to 10 for the AC motor, the desired proportion for the rotational speeds of the propellers will be achieved as the AC generator and the AC motor are directly connected by the electrical power connection and they have the same frequency.

FIG. 2 shows a schematic diagram of another exemplary embodiment wherein a main rotating power unit 40 that is for example a two-stroke diesel engine drives a propulsion AC generator 42, whose rotor is fastened to an output shaft of the rotating power unit 40. A main propeller 44 of the vessel is fastened onto another output shaft 46 of the rotating power unit 40. Though the generator 42 and the propeller 44 are on the opposite sides of the rotating power unit 40 in the embodiment shown in FIG. 2, the generator may also be on the same side of the rotating power unit 40 as the propeller 44. Opposite to the main propeller 44 is arranged a second propeller 48 which is driven by an AC motor 50. The main or the forward propeller 44 is fitted with a bearing to the hull of the vessel.

The second or the aft propeller 48 is supported with the AC motor 50 on the hull of the vessel or attached to the rudder arrangement of the vessel. The forward propeller 44 and the aft propeller 48 are arranged to operate in a contra rotating propulsion (CRP) mode that is known to those skilled in the art. The propulsion AC generator 42 is connected to the AC motor 50 by an electrical power connection 52.

A circuit breaker 54 is arranged on the electrical power connection 52 by which the connection between the AC motor 50 and the propulsion AC generator 42 can be switched on and switched off. When the circuit breaker 54 is switched on, the AC motor 50 and the propulsion AC generator 42 have the same frequency. Their rotational speed as well as the rotational speed of the aft and the forward propellers are effected by the frequency.

Two auxiliary rotating power units 56 and 58 are coupled to two generators 60 and 62, which are via electrical power connections connected to a main switchboard or a main bus 64 of the ship. The switchboard 64 is connected to the electric distribution mains of the vessel, and the generators 60 and 62 supply the electric power to the consumers.

The auxiliary rotating power units 56 and 58 are for example four-stroke diesel engines having lower power than the rotating power unit 40. The auxiliary rotating power units 56 and 58 can be located in a suitable space in the hull of the vessel and they need not be positioned near the propellers 44 or 48 of the vessel or near the other consumers of the electric power.

A frequency converter 66 is coupled between the main switchboard 64 and the junction 68 of the electrical power connection 52 via an electrical power connection 70 and via an electrical power connection 72 connecting, respectively. There is a second circuit breaker 74 between the main switchboard 64 and the frequency converter 66 and a third circuit breaker 76 between the frequency converter 66 and the junction 68.

The propulsion system shown in FIG. 2 can be operated and controlled in several ways depending on the operation mode. Referring to FIGS. 3 to 6, several operation modes will be clarified. When applicable the same reference numbers will be used the in FIGS. 3 to 7 as are used in FIG. 2.

FIG. 3 shows the FIG. 2 embodiment in a maneuvering mode where the propulsion power is generated by the auxiliary rotating power units 56 and 58 whereas the main rotating power unit is not working. The propulsion power is generated by the aft propeller 48 as illustrated by the arrow 82 and the forward propeller is at a standstill. Thus the AC generator 42 is not generating electric power and the circuit breaker 54 is switched off.

The circuit breakers 74 and 76 are switched on and the electric power is supplied to AC motor 50 via the frequency converter 66 and via the main switchboard from the generators 60 and 62. The flow of electric power is illustrated by the arrows 80. The power and frequency supplying the electric energy to the AC motor 50 are controlled by the frequency converter 66. The propulsion power of the aft propeller is limited to the capacity of the AC generators 60 and 62, and in practice to the capacity of the frequency converter 66.

FIG. 4 shows the FIG. 2 embodiment in a first start-up mode where the propulsion power is generated by the auxiliary rotating power units 56 and 58 and the main rotating power unit 40. The aft propeller 48 and the forward propeller 44 function in the CRP-mode as is illustrated by the arrows 82 and 84.

Though the main rotating power unit is functioning, the AC generator 42 is not generating electric power and the power circuit breaker 54 is switched off. The circuit breakers 74 and 76 are switched on and the electric power is supplied to ac motor 50 via the frequency converter 66 and via the main switchboard from the generators 60 and 62. The flow of electric power is illustrated by the arrows 80. The power and frequency supplying the electric energy to the AC motor 50 are controlled by the frequency converter 66.

In this mode the speed of the aft propeller 48 can be controlled independent of the speed of the forward propeller 44. The main rotating power unit is working on a lowered power level, such as for example 25% of its nominal power and the propulsion power of the forward propeller is for example about 25% of its total power (e./., plus/minus 10 percent).

The propulsion power of the aft propeller is limited to the capacity of the AC generators 60 and 62, and in practice to the capacity of the frequency converter 66.

FIG. 5 shows the FIG. 2 embodiment in a second start-up mode where the propulsion power is generated by the auxiliary rotating power units 56 and 58 and the main rotating power unit 40. The AC motor is supplied both from the AC generators 50 and 62 via the frequency converter 66 and from the AC generator 42 via the electrical power connection 52. The aft propeller 48 and the forward propeller 44 function in the CRP-mode as is illustrated by the arrows 82 and 84.

The main rotating power unit is functioning and the AC generator 42 is generating electric power and the power circuit breaker 54 is switched on. The circuit breakers 74 and 76 are switched on and the electric power is supplied to AC motor 50 via the frequency converter 66 and via the main switchboard from the generators 60 and 62.

The flow of electric power from the AC generators 60 and 62 is illustrated by the arrow 80, the flow of electric power from the AC generator 42 is illustrated by the arrow 86, and the flow of electric power to the AC motor 50 is illustrated by the arrow 88. The power and frequency supplying the electric energy to the ac motor 50 is controlled by the frequency converter 66. In this mode the power supply to the AC motor 50 is gradually moving from the AC generators 60 and 62 to the ac generator 42 by adjusting the output power of the AC generator 42 and controlling the frequency converter correspondingly.

The main rotating power unit is working on a lowered power level, such as for example 25% of its nominal power and the propulsion power of the forward propeller is again about 25% of its total power in order to keep the propulsion power levels of the aft and the forward propellers in the allowed limits. The propulsion power of the aft propeller is limited to the capacity of the AC generators 60 and 62, and in practice to the capacity of the frequency converter 66 and a proper CRP function involves the propulsion powers of the forward and aft propellers being in a desired proportion, such as for example between 20:80 and 50:50.

FIG. 6 shows the FIG. 2 embodiment in a third start-up mode where the propulsion power is generated by the main rotating power unit 40. This also illustrates a full speed and full power propulsion where both propellers are functioning by the power of the main rotating power unit 40 and the AC generator 42 is directly connected to the AC motor 50.

The circuit breaker 54 is switched on, whereas the circuit breakers 74 and 76 are switched off, and AC motor 50 is totally supplied from the ac generator 42 via the electrical power connection 52. The aft propeller 48 and the forward propeller 44 function in the CRP-mode as is illustrated by the arrows 82 and 84.

The flow of electric power to the AC motor 50 is illustrated by the arrows 90. During the third start-up mode the power of the main rotating power unit is increased from a lowered level, such as for example 25 towards its nominal power. In order to have an efficient CRP-function the propulsion powers of the forward and aft propellers are in a desired proportion, such as for example between 20:80 and 50:50.

As the AC generator 42 is directly connected to the AC motor 50 via the electrical power connection 52, the AC generator 42 and the AC motor have the same frequency. When they both are synchronous machines, their rotational speeds differ from each other on the basis of the pole numbers of the machines. Correspondingly, the rotational speeds of the aft and forward propellers differ on the same way as they are fastened to the shafts of the machines.

FIG. 7 shows the FIG. 2 embodiment in a fourth mode where the propulsion power is generated by the main rotating power unit 40. The main rotating power unit 40 is functioning at full power and the AC generator 42 is directly connected to the AC motor 50. The circuit breaker 54 is switched on and also the circuit breakers 74 and 76 are switched on and the frequency converter 66 is connected between the main switchboard 64 and the junction 68 of the line 52.

The AC motor 50 is totally supplied from the AC generator 42 via the electrical power connection 52. In addition the AC generator 42 is also supplying power to the main switchboard 64 as controlled by the frequency converter 66.

The flow of electric power to the AC motor 50 is illustrated by the arrow 94, the flow of electric power from the AC generator 42 is illustrated by the arrows 92, and flow of electric power to the main switchboard 64 is illustrated by the arrows 96. The propulsion power is lowered by the amount that is supplied to the main switchboard 64. In other respects the CRP functions as explained in the connection of FIG. 6.

FIG. 8 shows a schematic diagram of another exemplary embodiment wherein a main rotating power unit 2 that is for example a two-stroke diesel engine drives a propulsion AC generator 4 via a gear 20, which is coupled to a first output shaft 22 of the rotating power unit 2. A second gear 24 is coupled to the second output shaft 26 of the rotating power unit 2. The propeller 6 is connected to the secondary shaft 28 of the gear 24.

Opposite to the main propeller 6 is arranged a second propeller 12 which is driven by an AC motor 8. The main or the forward propeller 6 is fitted with a bearing to the hull of the vessel. The second or the aft propeller 12 is supported with the AC motor 8 by the hull of the vessel or by the rudder arrangement of the vessel.

The forward propeller 6 and the aft propeller 12 are arranged to operate in a contra rotating propeller (CRP) mode that is known to those skilled in the art. The AC generator 4 is connected to the AC motor 8 by an electrical power connection 14. The AC motor 8 and the AC generator 4 have the same frequency. The pole numbers of the AC generator 4 and AC motor 8 are defined so the rotation speed of the second propeller 12 is between for example 95% and 150% of the rotation speed of the first propeller 6.

FIG. 9 shows a schematic diagram of another exemplary embodiment wherein a main rotating power unit 2 that is for example a two-stroke diesel engine drives a first propeller 6 connected to its output shaft 10. Another rotating power unit 30 rotates the AC generator 32 which is coupled to the output shaft 33 of the rotating power unit 30. The rotating power unit 30 may be separate from the main rotating power unit 2 or it may be a turbocharger utilizing the exhaust gases of the main rotating power unit.

Opposite to the main propeller 6 is arranged a second propeller 12 which is driven by an AC motor 8. The main or the forward propeller 6 is fitted with a bearing to the hull of the vessel. The second or the aft propeller 12 is supported with the AC motor 8 by the hull of the vessel or by the rudder arrangement of the vessel.

The forward propeller 6 and the aft propeller 12 are arranged to operate in a contra rotating propeller (CRP) mode that is known to those skilled in the art. The AC generator 4 is connected to the AC motor 8 by an electrical power connection 34.

The AC motor 8 and the AC generator 4 have the same frequency. The pole numbers of the AC generator 4 and AC motor 8, and the rotational speeds of the main rotating power unit 2 and the other rotating power unit 30 are defined so the rotation speed of the second propeller 12 is between for example 95% and 150% of the rotation speed of the first propeller 6.

The arrangement disclosed herein may have many embodiments. Some of them are hereby mentioned as examples. The AC motor and/or the AC generator have one or more independent windings which can utilized as known in the art. The AC motor and AC generator and the electrical power connections can comprise three or more electrical phases.

The first propeller and second propeller can be coaxially aligned in the CRP arrangement. The propellers may also be slightly offset so that they are not exactly on the same line or so that they are slightly inclined.

The AC generator may include one or more separate generators. If there is not available a large enough generator, it is possible to alternately use two or several generators coupled in parallel and rotated by the rotating power unit. The generators can be connected and disconnected depending on the power demand.

It is possible that the AC generator only is driven by the same rotating power source as the first propeller. It also is possible to divide the main rotating power unit into smaller units, whereby one of them rotates the first propeller and the other rotates the AC generator connected to the AC motor of the second propeller. The smaller rotating power units can be positioned more freely in the vessel and the space will be used in a more optimized manner. The smaller power unit can be a gas turbine, whereby it is possible to adjust the proportion of the rotational speeds of the propellers.

The second propeller may be fixed to the hull or to the rudder of the vessel. The second propeller may be of a pod type propeller. It may also be of an azimuth type, which may be used for steering the vessel.

Different kinds of AC machines are possible both for the AC motor and for AC generator. The AC machine may for example be induction or synchronous machines. The synchronous machines may be excited by electrically controlled magnets or by a permanent magnet.

When an induction motor is used, the slip of the induction type motor is changed when the excitation of the synchronous type generator adjusted. By this way power relative to speed can be slightly adjusted. In a case where the generator is an induction type slip ring generator, where a rotor circuit includes external controllable resistors or the frequency converter enables control of the relative speed of the rotor and stator, it is possible to adjust further the slip of the generator. By this method the slip could for example be adjusted approximately ±20%. This generator can then drive an induction or synchronous motor with adjustable frequency. By both of these methods the load of the rotating power unit can be adjusted to be more optimal, for example, for the rotating power unit's fuel consumption.

Both the first propeller and the second propeller may have fixed pitch or controllable pitch.

It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein. 

1. An arrangement for supplying propulsion power to a contra rotating propellers (CRP) propulsion system in a marine vessel, which arrangement comprises: a rotating power unit for driving a first propeller; an AC motor for driving a second propeller, whereby the second propeller is configured for rotation in an opposite direction relative to rotation of the first propeller; and an AC generator configured to be driven by the rotating power unit, the AC generator being electrically connected to the AC motor, wherein a rotation speed of the second propeller will be between 95% and 150% of a rotation speed of the first propeller, wherein the AC motor and the AC generator have the same electrical frequency, and wherein a pole number of the AC generator is from 2 to 40, and a pole number of the AC motor is from 2 to 40 so that a ratio of the pole numbers of the AC motor and the AC generator is from 0.05 to
 20. 2. An arrangement according to claim 1, wherein a diameter of the second propeller is smaller than a diameter of first propeller.
 3. An arrangement according to claim 1, wherein a distance between the first propeller and the second propeller is less than twice a diameter of the first propeller.
 4. An arrangement according to claim 1, wherein a power proportion of the first propeller and the second propeller ranges between 50:50 and 80:20.
 5. An arrangement according to claim 1, wherein the AC motor is supplied by a second electrical power source which is electrically connected in parallel to the AC generator.
 6. An arrangement according to claim 1, wherein the AC generator supplies electric power to a second electrical consumer which is electrically connected to the AC generator.
 7. An arrangement according to claim 5, wherein the second power source comprises: a frequency converter, the second power source being electrically connected via a switch to the AC motor.
 8. An arrangement according to claim 5, wherein the second power source is switchable to the AC motor and/or the AC generator when changing power distribution between the first propeller and the second propeller.
 9. An arrangement according to claim 5, wherein an electrical power connection between the AC motor and the AC generator comprises: a line breaker, the AC generator and the second power source being configured to be alternatively switchable to the AC motor when maneuvering a vessel.
 10. An arrangement according to claim 1, wherein the AC motor and/or the AC generator have one or more independent windings.
 11. An arrangement according to claim 1, wherein the AC generator comprises: one or more separate generators.
 12. An arrangement according to claim 1, comprising: one or more mechanical gears between the first propeller and the rotating power unit driving the first propeller.
 13. An arrangement according to claim 1, comprising: one or more mechanical gears between the AC generator and the rotating power unit driving the AC generator; and/or a gear unit arranged between the second propeller and the AC motor.
 14. An arrangement according to claim 1, wherein the second propeller is of an azimuth type.
 15. An arrangement according to claim 1, in combination with a marine vessel comprising: first and second propellers. 